ROMEOVILLE, Ill. — Brandon Williams arrived at an Amazon fulfillment center here, about an hour outside of Chicago, around 7:30 a.m. on Wednesday, one of thousands across the country who turned up for the company’s first Jobs Day. While he appeared to wilt slightly during the five hours he waited before an M.C. summoned him for a tour, his enthusiasm did not wane.

“What’s not great about a company that keeps building?” he said, seated in a huge tent the company erected in the parking lot as a kind of makeshift waiting room.

The event was a vivid illustration of the ascendance of Amazon, the online retail company that, to a far greater extent than others in the tech industry, has a seemingly insatiable need for human labor to fuel its explosive growth.

Like other tech giants, Amazon is recruiting thousands of people with engineering and business degrees for high-paying jobs. But the vast majority of Amazon’s hiring is for what the company calls its “fulfillment network” — the armies of people who pick and pack orders in warehouses and unload and drive delivery trucks, and who take home considerably smaller incomes.

The event on Wednesday, held at a dozen locations including Romeoville, Ill., was intended to help fill 50,000 of those lower-paying positions, 40,000 of them full-time jobs.

Those high-low distinctions did not seem to bother the attendees of the jobs fair, many of them united in the conviction that Amazon represented untapped opportunity — that a foot in the door could lead to a career of better-compensated, more satisfying work, whether in fulfillment, I.T., marketing or even fashion.

Mr. Williams, a military veteran studying computer network security at a nearby community college, said he hoped to eventually work his way up to an I.T. job with Amazon. But even those whose ambitions were more in line with the vast majority of available jobs could not hide their excitement.

Victor Salgado, who makes $9 per hour with no benefits doing maintenance at an aerosol company and spent three years before that grouting floors, said he was attracted by the promise of an upgrade in pay and benefits. He said he would be willing to “do anything” at Amazon.

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The vast majority of Amazon’s hiring is for what the company calls its “fulfillment network.”CreditLyndon French for The New York Times

“A record-breaking 20,000 applications were received on this day alone,” said John Olsen, an Amazon human resources vice president, “with thousands of job offers extended to candidates and more to come in the next few days.”

Filling so many jobs is challenging, which partly explains the jamboree at Amazon’s warehouses on Wednesday. The company’s warehouse jobs, which typically pay $12 to $15 an hour, have a reputation as physically demanding and repetitive, with high rates of burnout. Amazon has successfully resisted union organizing that might introduce more worker protections.

Amazon has said its wages and benefits are attractive. Five years ago, it introduced a program called Career Choice that pays tuition costs for employees seeking training to join higher-paid professions, like airline mechanics, medical lab workers and computer-aided design technicians.

Jack Chasteen, who recently finished high school and is still living at home, said he had been planning to seek a job as a pizza deliveryman but that his parents urged him to turn up at Jobs Day instead.

“They said it would be a good building block for stuff in the future,” he said, citing Amazon’s tuition assistance program, which he hoped might help him pursue a nursing degree.

It is indisputable that the growth of e-commerce, which Amazon captures a big chunk of, has devastated many physical retailers, leading to store closures and layoffs. There is debate, though, about how much the corresponding expansion of jobs related to e-commerce has offset the decline in employment at physical stores.

Amazon’s remarkable head count growth stands in sharp contrast to its image as a job killer. It was the fastest American company in history to employ 300,000 people globally, crossing that threshold last year, its 20th as a public company, according to a paper published this year by the Progressive Policy Institute, a left-leaning think tank. Walmart reached that milestone 21 years after it went public, the paper said.

Amazon now has more than 382,000 employees globally. In January, it vowed to create 100,000 jobs over the next 18 months in the United States. It increased its domestic work force from 30,000 employees in 2011 to over 180,000 at the end of 2016.

The figures do not include the thousands of seasonal workers that join the company to help it with the crush of holiday shopping. Some come in R.V.s as part of a group Amazon called CamperForce. The company pays for their campsites.

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Many of the people who showed up were united in the conviction that Amazon represented untapped opportunity.CreditLyndon French for The New York Times

Amazon hired 30,000 new employees in its last quarter alone. Facebook, in contrast, employed just under 21,000 people at the end of June, while Alphabet, the parent company of Google, employed about 76,000.

Arun Sundararajan, a professor of information, operations and management sciences at New York University’s Stern School of Business, said Amazon’s employment needs are unique among tech companies.

“Their business has always been a meld of the digital and physical,” he said. “Retail is very different from digital products or music or social networking. The other tech platforms are, at their core, selling tech products.”

But there are questions about how long Amazon’s fulfillment jobs will exist, as robots and other forms of automation become more capable at doing the jobs that now require humans.

Amazon is more aggressively using robots to help make the operations inside its warehouses more efficient. For now, the company said machines are not replacing people. Instead, they mostly move large shelves of merchandise to stations where orders are manually picked.

Many academic researchers and start-ups are working on robots that have the dexterity to pick orders automatically. Amazon sponsors a competition to encourage engineers to build more advanced warehouse robots.

When those technologies are perfected, the employment picture inside Amazon’s warehouses could look very different. That day could be a decade or more away, though.

“While the digital disruption is destroying the traditional retail business model,” Dr. Sundararajan said, “the Amazon model that replaces it will continue to live in the physical world and require human labor for the foreseeable future.”

As for Mr. Williams, he later sent a text message to say that he was not hired on the spot on Wednesday, a possibility that Amazon had dangled before workers. On the plus side: he was told there was still a chance to get a job at the center through a temporary staffing company.

Correction:

An article on Thursday about Amazon’s Job Fair misstated the hourly pay that one of the event’s attendees, Victor Salgado, makes at an aerosol company. It is $9 per hour, not $10.75.